TORONTO( Reuters)-As students in Canada's most populous province return to mask-free classes after two years on Monday, one Ontario school board is facing backlash for defying the province's decision to drop masks, potentially setting the stage for a clash on a contentious pandemic issue. The mask mandate and other pandemic measures have become a lightning rod in Canada for an anti-government movement, sparking a three-week protest in capital Ottawa last month. The Hamilton-Wentworth District Sc